The Tversky index, named after Amos Tversky, is an asymmetric similarity measure on sets that compares a variant to a prototype. The Tversky index can be seen as a generalization of the Sørensen–Dice coefficient and the Jaccard index. For sets X and Y the Tversky index is a number between 0 and 1 given by Here, denotes the relative complement of Y in X. Further, are parameters of the Tversky index. Setting produces the Tanimoto coefficient; setting produces the Sørensen–Dice coefficient. , ,